Refractive Errors

About 120 million people in the United States wear eyeglasses or contact
lenses to correct nearsightedness, farsightedness, or astigmatism. These vision
disorders--called refractive errors-- affect the cornea and are the most common
of all vision problems in this country.

Refractive errors occur when the curve of the cornea is irregularly shaped
(too steep or too flat). When the cornea is of normal shape and curvature, it
bends, or refracts, light on the retina with precision. However, when the curve
of the cornea is irregularly shaped, the cornea bends light imperfectly on the
retina. This affects good vision. The refractive process is similar to the way a
camera takes a picture. The cornea and lens in your eye act as the camera lens.
The retina is similar to the film. If the image is not focused properly, the
film (or retina) receives a blurry image. The image that your retina "sees" then
goes to your brain, which tells you what the image is.

When the cornea is curved too much, or if the eye is too long, faraway
objects will appear blurry because they are focused in front of the retina. This
is called myopia, or nearsightedness. Myopia affects over 25 percent of all
adult Americans.

Hyperopia, or farsightedness, is the opposite of myopia. Distant objects are
clear, and close-up objects appear blurry. With hyperopia, images focus on a
point beyond the retina. Hyperopia results from an eye that is too short.

Astigmatism is a condition in which the uneven curvature of the cornea blurs
and distorts both distant and near objects. A normal cornea is round, with even
curves from side to side and top to bottom. With astigmatism, the cornea is
shaped more like the back of a spoon, curved more in one direction than in
another. This causes light rays to have more than one focal point and focus on
two separate areas of the retina, distorting the visual image. Two-thirds of
Americans with myopia also have astigmatism.

Refractive errors are usually corrected by eyeglasses or contact lenses.
Although these are safe and effective methods for treating refractive errors,
refractive surgeries are becoming an increasingly popular option.